Each state's laws determine how much child support a non-custodial parent must pay after a divorce, and the rates and method of calculation vary between states. These payments are intended to pay for a child's normal expenses, such as housing, food, clothing and education. Though courts frequently use the guidelines to set child support amounts, courts do not have to follow them in cases where they would not be appropriate, such as when a child needs special medical care because of a disability.

Percentage of Income

Some states use a child support model called the percentage of income model. In this model, the state sets child support as a percentage of the non-custodial parent's income, increasing the percentage as the number of children increases. For example, Texas non-custodial parents generally pay 20 percent of their income for one child, increasing to over 40 percent for six or more children. New York's guidelines require non-custodial parents to pay 17 percent for one child, increasing to at least 35 percent for five or more children.

Income Shares

More states now use an income shares model or Melson formula to calculate child support. The income shares model sets child support based on the proportion of income each parent contributes, attempting to ensure the child receives the same percentage of each parent's income as he would have received if the parents lived together. For example, if the custodial parent makes 60 percent of the combined parental income, the non-custodial parent will only pay 40 percent of the child support obligation assigned by the state. The Melson formula is a version of the income shares model, but it contains steps designed to ensure each parent's basic needs are met.

Income

All states base child support, at least in part, on parental income. However, income includes more than just the wages a parent takes home from his paycheck. Income can also include commissions, tips, bonuses, interest, royalty income, self-employment income, rental income, gifts and unemployment benefits. Generally, income includes almost any asset coming into a parent's household, but it does not include a new spouse's income. Thus, if a father who makes little money marries a wealthy new wife, his child support amounts are likely to be set at a low level since they are based on his income rather than his wife's.

Modifying Child Support

Since parental income and circumstances change over time, child support amounts may also change. State law varies, but many states allow parents to ask the state child support agency or court for a modification after a certain period of time. For example, Washington allows modifications after three years or if there has been a significant change of circumstances since the last child support order was issued. State rules might not allow modifications unless the support amount changes by a certain percentage. For example, Tennessee requires at least a 15 percent change between the former child support amount and the new child support amount unless circumstances, such as the birth of additional children, require a change.

About the Author

Heather Frances has been writing professionally since 2005. Her work has been published in law reviews, local newspapers and online. Frances holds a Bachelor of Arts in social studies education from the University of Wyoming and a Juris Doctor from Baylor University Law School.